Electrochemical conversion cells, commonly referred to as fuel cells, which produce electrical energy by processing first and second reactants, e.g., through oxidation and reduction of hydrogen and oxygen. By way of illustration and not limitation, a typical polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane (PEM)) that is positioned between a pair of gas diffusion media (DM) layers and catalyst layers. A cathode plate and an anode plate (or bipolar plates BPP) are positioned at the outermost sides adjacent the gas diffusion media layers, and the preceding components are tightly compressed to form the cell unit.
The catalyst layers can be attached to the PEM forming a membrane electrode assembly (MEA). One method of forming an MEA involves depositing an electrode ink on the PEM by direct spraying or coating in a shim frame. Due to the creeping of the PEM when it becomes wet, this method is usually difficult to control. Alternatively, the electrode can be formed on a decal and transferred to the PEM. Typically, the powder catalyst and ionomer solution are dispersed in a mixed solvent which usually contains one or more alcohols and water in a specific ratio that depends on the type of catalyst. The mixture is then homogenized by ball-milling for 2-3 days before coating on a decal substrate. For shim coating, the catalyst loading can be controlled by the thickness of the shim; for the Mayer rod coating, the catalyst loading can be controlled by the thread number. Multiple coatings can be applied for higher catalyst loading, with a drying step in between every two consecutive coatings. After the catalyst/ionomer coated decal dries out, the catalyst/ionomer is then transferred onto a PEM by hot press to form an MEA. The anode and cathode can be hot-pressed onto a PEM simultaneously. The pressure and time for the hot press may vary for different types of MEAs. Alternatively, the catalyst/ionomer ink can be coated on a diffusion media, followed by hot press onto the PEM upon its drying out.
An electrode ink typically contains ionomer, organic solvents such as isopropyl alcohol, ethanol, etc. and electrocatalyst. Additional materials can be incorporated into the electrode ink to increase the electrode performance robustness. Ionic conducting components can be incorporated into the electrode ink, if desired. Hydrophobic particles, for example, PTFE, can be incorporated into the electrode ink to improve the electrode water management capability, if desired. Graphitized or amorphous carbon powder or fiber, other durable particles, or other electrocatalysts like Pt supported on carbon can also be incorporated into the electrode ink to increase the electrode water storage capacity, if desired.
Carbon-based electrodes, such as high surface area carbon (HSC) and graphitized carbon, typically include carbon, which can function as the catalyst and/or the catalyst support, an optional metal catalyst, and ionomer as the binder and ion conductor.
When carbon-based electrodes are used in the MEA, mud-cracking, non-uniform coating, and decal transfer are difficult challenges, particularly when an ultra-thick electrode (e.g., about 12 microns or more) is needed. A mud-cracked or non-uniform electrode has a detrimental effect on the performance and durability of the MEA.
Methods of reducing mud-cracking have been developed, including using a high boiling point solvent, adding an acid to the catalyst ink, and mixing catalyst with carbon fibers. Generally, these additives are added to the electrode ink before ball-milling. For example, the high boiling point solvent can be ethylene glycol, glycol ethers or glycol esters such as propylene glycol butyl ether (PGBE), etc. The additive acid can be diluted nitric acid. However, the use of the high boiling point solvent and acid additives have potential poison effects on the electrode, and the use of carbon fiber increases the risk of cell shorting and cross-over.
Therefore, there is a need for a method of making MEAs using non-noble metal catalyst electrodes, such as carbon-based electrodes, or thick electrodes without mud-cracking or increasing the risk of shorting or cross-over.